That Sinking Feeling

I suppose most former little boys can remember having the occasional dread of disappearing into the ground. We all go through it don’t we, a phase of thinking we will be swallowed up by sinking sand, sucked into a bog or vanishing down a crack in the ground during an earthquake. For a while it seemsall consuming, anyone would think you’d find sinking sand on every street corner.

As adults most of us have occasional fears of planes falling out of the sky or ships sinking to bottom of the sea. I remember a particularly terrifying trip on a rust bucket they called a ferry from Sumbawa to Lombok which prompted several members of our expedition to take rather frequent rests (in the rest room). It was evenmore scary than Donald Trump.

We all have a fundamental need to feel supported and the most basic support is to have our feet on firm ground.

So bear a thought for a woman I met recently. As she walked into her bedroom one day her heart sank, but so did her feet, her legs, her body and her head didn’t get left behind either. She lost her balance and dropped the apple she was carrying which, strangely, rolled to the centre of the room and what had for some time only been a mere hint that something was amiss suddenly became abundantly clear.

Her bedroom floor looked like a giant wok. Rather like Indonesia’s abundant plastic being drawn into the Great Pacific Garbage Patch, the contents of her bedroom were on the move – towards the centre of the room.

Her floor was sinking.

I have seen a quite number of rooms like this over the years, it is a surprisingly common problem here in Indonesia. In some cases the phenomenon of sinking floors is claimed to be due to paranormal activity which, in reality, means it is due to a genetic disposition suffered by building contractors that stimulates an insane desire to save trifling amounts of money regardless of the fact that this may result in major expense or catastrophe being incurred later on.

Generally, however, such problems are the result of inadequate or incompetent building design and/or construction. A group of villas we regularly work in are typical. The villas are generally well designed and built, the structures are sound and the finishing is good but they all have two significant problems – flat concrete roofs that leak and unstable floors. It is very unfortunate because the floors were originally finished in beautiful terrazzo panels with coconut wood joints. The floor panels have a tendency to move by varying degrees and in many cases moisture coming up between the joints has seriously stained the terrazzo. The problem is that the terrazzo lies directly on top of compacted rubble with no concrete or steel mesh to reinforce it. The problem can only be resolved by taking up the floors and starting again. In fact this is not difficult and a new sub floor is not expensive but the tragedy is in the loss of the expensive finishing – in this case the beautiful terrazzo.

So what can make a floor unstable.

  • Poor compaction of the ground or fill under the floor.
  • Underfloor fill being expansive clay that will expand and contract between the wet and the dry season.
  • Failure of foundation retaining walls if the floor is set above ground level.
  • Inadequate or even lack of a subfloor, usually a concrete layer that forms the foundation for the floor finish.
  • Poor drainage that allows water to flow through the ground under the floor and wash out the fill.
  • Organic materials (such as tree branches or othervegetation) buried in the earth used to fill the site before construction.
  • Tree roots growing under the floor.

Problems can start before the house is built with bad filling of an uneven site. It is best to level a building site before filling then putting in a 1 metre thick layer of crushed stone or gravel under the area where the building is to be constructed.

If the site is filled it is a very good idea to let it settle for a year before construction starts. If you buy land but don’t plan to use it just yet fill it now so it can be settling ready for construction later. Remember that any organic material in the fill (such as tree trunks) will rot away leaving voids in the ground that can lead to subsidence later.

When building a house it is common practice to build stone foundation walls then cast concrete ground beams on top of these walls, these beams will tie the bottom of structural columns together and form the base of the building’s structural frame. The areas between the ground beams are usually filled (it might be soil, clay, gravel, sand, rubble or rocks) and then a thin layer of lean mix concrete (lean – not much cement) sometimes with a light steel mesh is cast to form the subfloor.

This is common practice in Indonesia but, as many people have found, it is generally inadequate. The layer of weak concrete can crack easily and usually cannot withstand even small stresses that may lift or subside the floor. In addition the steel may rust, termites may easily penetrate the floor and, not being sufficiently, waterproof moisture can rise through it from the ground beneath. The joints between the weak concrete subfloor and the surrounding ground beams are particularly vulnerable areas.

If the house is raised up above aground level the ground under the house will have to be contained by some kind of retaining wall. If this retaining wall is not correctly designed and is not strong enough to contain the earth under the house the weight of the building may break the retaining walls allowing the earth to spread and the house to subside. I recently saw a Joglo with this problem.


Poor drainage can be disastrous for any building. As we all know rainfall here can be very heavy and can undermine the foundations of buildings. You may also get water flowing underground that you are not aware of that can wash out the ground under the floors and result in subsidence. You need to make sure that drainage around your buildings is well designed and managed, this is particularly important if your building is on a slope or near rice fields.

Waterproofing Membranes

A waterproofing membrane should be installed under the concrete floor to prevent capillary action drawing moisture up through the floor from the ground beneath. The membrane should be a single piece, if you are lining a fishpond using several sheets overlapped at the joins it will not work – it won’t work under a floor.Unfortunately very large single sheets of plastic membrane and very difficult to come by in Indonesia.

Tree roots

As most people know the strength of tree roots is amazing. You only have to go and have a look at places such as Ankor Wat to see that even substantial construction cannot hold back nature. Tree roots can do extensive damage to a house or any structure for that matter. If tree roots get close underneath the ground beams or floors of your house they can do a lot of damage so it is a good idea to go around your house and check for roots of trees and shrubs. If you find roots cut them off to stop them growing.

Repairing sunken floors

As always in repair work it is important to understand and address the fundamental causes of a problem. It may be that floor subsidence is simply due to settlement after the building has been completed and, once settled, the ground will become stable and won’t move anymore. Experience says that this is only the case in a few situations. In the majority of cases once the floor has started to move it will probably continue to move.

It may only be possible to identify the cause of floor subsidence by removing the floor and seeing what is underneath. We need to consider the possible causes I have listed, determine which is relevant and then engineer solutions that correctly address the problem. Don’t take anything for granted.

New Building

When building from scratch it may be a good idea to construct the floors in a different way. Instead of casting floor panels between the ground beams of the building it is easy to cast the floor as a single slab right through the house across the top of the ground beams. Such a slab should be full strength concrete with a good solid steel mesh used to reinforce it, two layers of mesh in the concrete (this is always used in upper concrete floors) considerably increases strength and is a good idea if there are cavities or other weaknesses in the ground below the floor. This design may cost more (not a lot more) but will provide a stronger more stable floor and will provide an effective barrier against moisture and termite penetration.

Phil Wilson

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